ocfs2/dlm: Use ast_lock to protect ast_list

The code was using dlm->spinlock instead of dlm->ast_lock to protect the
ast_list. This patch fixes the issue.

Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>
Acked-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>